Is it legal / mutar to use a free PDF version of a book, instead of buying a hard copy?
This is required reading for work, which the company will gladly pay for, it’s just easier to read it on my computer than pick up a hard copy.
If the PDF version is legally distributed then it is permitted to use it. Some people upload copyrighted material, and doing so is illegal and forbidden according to halachah. However, once on the Internet it can be hard to know which material is there legitimately and which isn’t.
The general advice is not to use “sketchy” sites, but only trusted websites that won’t be breaking the law.
Many books are available as “ebooks” for which you have to pay, and of course this is fine, being a sold version of the book that the publisher distributes electronically.